Why have I got an extra vertex when exporting to ply format?
This script will set all vertex color alphas to 1 on the active object (I couldn't find a way to do it through the UI). Select your object, make sure you're in Object Mode, paste it in the text editor and hit run.
import bpy
assert bpy.context.mode == 'OBJECT'
for vc in bpy.context.active_object.data.vertex_colors:
    for c in vc.data:
        c.color[3] = 1

However a vert might still get split up if there are different vertex colors at the different corners that meet it, like this:

This will export with 16 verts. There's not really anything you can do about that, I think that's just how .ply works.